Electrical power from AC mains is used to provide a source of energy for numerous devices. Unfortunately, the electrical power provided by AC mains may be interrupted either partially (e.g., brown out) or wholly (e.g., power failure). Recognizing this problem, prior art power supply circuits have included voltage level detection and time period detection so as to determine when power is lost and whether the duration is temporary or more permanent. These prior art circuits either required a separate Vcc power supply or used excessive amounts of power from the AC mains. Accordingly, there is a strong need in the art for a simple, inexpensive and energy efficient manner for detecting the loss of the AC mains in a power supply circuit.